REE patterns for apatites from Proterozoic phosphatic metasediments, Finland

Abstract

Several occurrences of uranium- and apatite-bearing metasediments are known in the Precambrian of Finland. The characteristic rock types in the occurrences: metaphosphorite, apatite-bearing carbonate and calc-silicate rocks and apatite-quartz-feldspar gneiss have been interpreted as metamorphic derivatives of phosphorite and phosphatic sedimentary and pyroclastic rocks. Present modes of occurrence of uranium include incorporation into fluorapatite and into evenly disseminated uraninite crystals. Thirteen fluorapatite concentrates extracted from these metasediments were analysed for REE, U and Th by instrumental neutron activation. The concentrates averaged 113 ppm La, 118 ppm Ce, 21 ppm Sm, 4.8 ppm Eu, 13.7 ppm Yb, 485 ppm U and 2.0 ppm Th. The REE distribution patterns of the concentrates are reasonably similar and resemble the REE pattern of the apatite from the Tertiary Florida phosphate rock. A marine environment of deposition is suggested for the original phosphatic material.

Keywords

• REE
• U
• apatite
• metasediment
• Proterozoic
• Finland